The present invention relates to a system for the transmission of information by means of a remote control signal composed of first and second signal components at different frequencies, in which the information is represented by the time duration of the first signal, particularly for ultrasonic remote controls in television receivers.
It is known to transmit information by coding a signal serving as a carrier in accordance with such information. The coding may be effected, for example, by controlling the length of each time period during which the signal is being transmitted in dependence on the information (pulse duration modulation). In order to recover this information, the receiver must be provided with a decoding circuit which permits evaluation of the time duration of the signal.
Such a circuit is disclosed in the periodical "Funkschau" 1973, Issue No. 18, pages 675-677, for an ultrasonic remote control system for television receivers. In this known system, the ultrasonic signal emitted by a transmitter is composed of two immediately succeeding components at different frequencies, the components being coded with respect to both their frequency and duration.
The frequency of the first ultrasonic signal here determines the type of information, i.e. whether a channel is being selected, for example (switching function), or an analog function is being performed in the television receiver, while the time duration of the first ultrasonic signal determines the channel number or the type and direction of the analog function, i.e. the value of the information of the type indicated by the frequency. The second immediately following ultrasonic signal effects performance of the previously recognized function.
The evaluation of the time duration of the first ultrasonic signal is effected in the known circuit in that during the presence of the ultrasonic signal a multivibrator is switched on to produce pulses each of a defined period duration. These pulses are counted by an electronic counter which sets its counting state in dependence on the time duration of the ultrasonic signal, which practically constitutes the decoded information.